Re: NOS military radio antenna base MX-2799 AB-719/VRC on Ebay

From: Buzz (muttman@charter.net)
Date: Sun Apr 09 2006 - 21:58:49 PDT


The problem with running high power on the 524/246 is the heavy load on the DC
to DC power supply, especially the early radios with germanium power
transistors.

Buzz

On Sun, 09 Apr 2006 22:27:22 -0500, you wrote:
>The final in that radio (and the RT246) is rated for 100W dissipation. On
>low power, it can run indefinitely into any load. The radio is supposed to
>run at 35W on High power. If the PA is tuned correctly (as set by someone
>who knows how to work on those radios), it can handle poor antenna matches
>very well. As a reference, the SWR roughly indicates how much of the
>forward power is reflected (as reverse power) back from the antenna to the
>final amplifier, and is therein dissipated as heat in the tube.
>
>1:1 - zero
>1.5:1 - 4%
>2:1 - 11%
>3:1 - 25%
>4:1 -I don't remember but it's worse.
>
>If the radio is putting out 35 watts, the tube is likely burning up 35
>watts as heat to do that. (the total power into the tube is 70 watts) If
>you have a 3:1 SWR, then 25% of that 35W gets reflected back into the tube,
>adding 8.75 watts to the heat. Now the tube dissipates 43.75 watts. Almost
>half of what it is rated for. -So it's not as bad as it looks but for 4:1 I
>would leave it on low power.
>
>If your radio has been "hot rodded" and is putting out 50 or 60 watts, then
>the SWR best be kept below 2:1
>
>The tube used in there, in reality a conduction-cooled UHF triode, is very
>efficient at the low frequencies the radio is used at, the voltage on the
>tube is lower (only 650V I think) than it can take, and the things are
>really bulletproof. I think it's the CC version of the 2C39.
>
>After all of this, though, I would add that if transmissions are kept very
>short, the thing can run into any load.
>
>Some people have drilled a weep hole in the bottom of the adjustable
>section of the antenna to allow it to drain. Don't know how well that works.
>
>Some experts on the list may correct me on these matters.
>
>PJ
>
>
>Chance Wolf wrote:
>
>> ----- Original Message -----
>> From: "Patrick Jankowiak" <recycler@swbell.net>
>> To: "Military Vehicles Mailing List" <mil-veh@mil-veh.org>
>> Sent: Saturday, April 08, 2006 5:17 PM
>> Subject: Re: [MV] NOS military radio antenna base MX-2799 AB-719/VRC on Ebay
>>
>>
>>
>>>Kobs? We don't need no steenking knobs!
>>>
>>>Yeah there's no knob, but the SWR plot is <2 from 30 to 90 MHz.
>>
>>
>> Yeah. As compared to the MX-6707 which seems to always be 2+ even on Happy
>> Days. Supposedly the RT-524 can handle a continuous SWR of 4, but given the
>> price of finals I don't think I'd care to put it to the test!
>>
>> I was actually going to leave a meter permanently hooked up in the line so I
>> could always tell when my MX-6707 decided to become a swimming pool, but
>> never got around to it. I've had lots broil. At least now I give the knob
>> a spin before doing anything with the radio, and make sure the servo cycles
>> with a flick of the RT-524's band switch. I guess I should be keeping a
>> cover on the 6707s when not in use, but I never remember to do stuff like
>> that when it counts.
>>
>> (I had both mine fail in quick succession when I was on the way to San Jose
>> for the convention the one year, so I was out of comms for most of the trip.
>> Really annoying!)
>>
>>
>>
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>
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